This invention relates to systems and methods for fluid transport, as well as to oil country tubular goods for pipeline and downhole applications. More particularly, it relates to easily assembled tubular systems and methods for transport of fluids, such as oil, gas, potable water, slurries and the like, in which tubing sections are permanently interconnected in such manner as to have both interior sealing and adequate tensile strength for each given application.
Oil and gas pipelines, as typical examples of fluid transport lines, are usually interconnected by coupling individual sections together in end-to-end relation, often by welding and sometimes by end sections which mate together, with or without internal seals, and usually with threaded connections. Because the fluids being transported may be corrosive, at high temperature or high pressure, it is also common to use internal coatings or linings in the pipe. The internal coating or lining must have special characteristics, and therefore is typically expensive, but even so, the lining is often not free of defects or potential defects. Where the parent metal is thermally stressed, as by welding, the heat affected regions can immediately or latently develop defects, known as holidays. Similarly, where the parent metal is highly mechanically stressed, as by the radial interferences required for threaded pipe, immediate or latent cracking in the coating or lining can also occur at the stress zones. Thus the industry has emphasized the need for "holiday free" internally coated pipe connections. In recognition of the problems involved in attaining this goal, numerous techniques have been devised for repairing pipes and overcoming the damage done to coatings by the welding process.
Because pipelines are usually disposed horizontally, above ground or underground, they are not highly tensioned by a suspended mass. Thus, where high stresses exist they are typically the consequence of high internal pressures and thermal expansion and contraction coefficients. No loads of consequence are imposed axially on the couplings.
In downhole installations, however, substantial tensile stresses are applied in accordance with the length of the string and weight of the pipe, so threaded couplings are used where strings are to be assembled and disassembled. Where tubular goods are to be permanently installed, however, as in downhole installations in which pipe strings of limited length are encased in cement, then different interconnection systems, including those in accordance with the invention, become feasible. Here the requirements remain for easy assembly, maintaining the integrity of the coating or lining, and maintaining seals against high internal pressures and corrosive, high temperature gases or fluids. In addition, however, the assembly must withstand the mechanical tensile loads that are imposed. Where these requirements can be met, however, there are particular advantages to be derived from an ability to make connections quickly, because of the high costs of drilling rigs and drilling crews.